Rubbery-handsome, with a chipped front tooth, fashion-disaster bangs, and the eager dimples of a depraved gopher, Jim Carrey turns his face and body into a special effect — a human morph machine — in Dumb and Dumber. He’s playing a geek who thinks he’s hot stuff, and though we’ve seen this character before (Steve Martin practically invented it), Carrey, zigzagging between twinkly-eyed infomercial-pitchman bravado and sheer manic idiocy, does the postmodern smart-dumb clod with a new kind of whiplash abandon.

Dumb and Dumber, which features Carrey and Jeff Daniels as nitwits who journey cross-country to return a suitcase full of cash, is a pokey, hit-or- miss buddy comedy, a frayed string of gags posing as a movie. Carrey and Daniels squirt themselves with mustard and ketchup, perform a spaz rendition of ”Mockingbird,” and ride into Aspen on a tiny moped. Essentially, Dumb and Dumber is The Jerk with two jerks instead of one. Carrey, though, sustains a comic persona here in a way he couldn’t quite in Ace Ventura or The Mask. He does literal-minded doofdom with peerless enthusiasm — he’s like Robin Williams without the preciousness. In one of the film’s highlights, he fantasizes himself at home with the woman of his dreams (Lauren Holly). As the Cowsills’ ’60s love-schlock ditty ”The Rain, the Park & Other Things” bounces away on the soundtrack, we see the two caught in an angelic embrace — at which point Carrey flips up the back of her minidress, flashes a schoolboy leer, and then instantly returns to his ”tender” romantic gaze. Almost any other comic would have turned that moment into a full-fledged daffy routine. With Carrey, it’s just a grace note, but the look on his face lingers — wickedly. Unlike Jerry Lewis, he’s too quick to let you catch him acting dumb.

If Carrey is ever cast in a movie as witty as he is, it could be some sort of classic. But Dumb and Dumber, the best of his pictures so far, is still a middling vehicle. There may be limits to how much comic irony can be wrung out of the spectacle of two grown men acting like complete cretins. The funniest moment isn’t really a joke at all but an outrageous gross-out. Daniels, playing a character even stupider than Carrey’s, inadvertently drinks a cup of laxative and then, in a panic, rushes into the bathroom and well, goes (complete with graphic sound effects). Comedy doesn’t get much lower than this, but I defy anyone not to erupt into giggles watching Jeff Daniels’ face enact the five stages of bowel emergency. B-